Lithium-Air Batteries: An Overview

Yuan Zhong
December 3, 2011

Introduction

The technological revolution over the past centuries
consumes vast amount of energy and results in significant carbon
footprint. A dominate proportion of overall energy demand is attributed
to transportation sector which leads to various environmental problems,
such as urban air pollution. Approximately 80% of national CO emission
is accounted for transportations. [1] With foreseeable shortage of fuels
and emphasis on being green, electrification of road transportation is a
potential solution to both energy conservation and environmental
protection. The advancement of electric cars has been sluggish in the
past century due to the lack of suitable batteries. Lithium-ion
batteries are generally considered the potent candidate for electric
propulsion source in the near future. With an specific energy of 180
Wh/kg, though 5-fold higher than lead-acid batteries, a higher energy
storage capacity is still desired to further reduce total battery weight
on board and increase overall operation efficiency. [2] Li-air
batteries, which is theoretically proved to be of high energy density,
show a noticeable potential of being the future electric propulsion
source with excellent carbon footprint record. [2]

The metal-air batteries are usually defined as
batteries consist of metal-based anode and air-cathode which constantly
extract oxygen from the ambient air. Specific batteries are
characterized by the metal anode. For example, Li-air batteries refer to
those containing lithium metal as anode material. The first metal-air
battery is discovered by Leclanche in 1868. The nowadays commercialized
zinc-air batteries is developed by Heise and Schumacher in 1932. Li-air
batteries are first proposed in 1970s for their exceptionally high
specific energy and power which qualify as a potential energy source for
electric vehicle propulsion. [3] Non-aqueous Li-air batteries are first
reported to be rechargeable by K.M. Abraham in 1996. The theoretical
specific energy of Li-air batteries is calculated as 5,200 Wh/kg, or
equivalently, 18.7 MJ/kg including oxygen. [4] Since oxygen is
constantly drawn from air, specific energy is often quoted excluding
oxygen content. This theoretical specific energy is calculated to be
11,140 Wh/kg, or 40.1MJ/kg which is close enough to that of gasoline,
around 46MJ/kg. [4,5] Interest in Li-air batteries continues to grow
because of the high energy capacity which is promising when comparing
with gasoline. Due to engine inefficiencies, both gasoline and Li-air
battery are predicted to achieve a practical specific energy of 1,700
Wh/kg which is several folds higher than most of the existing battery
systems. [6]

Working Principles

A schematic basic non-aqueous Li-air battery cell, is
illustrated in Fig. 1. The cell comprises a Li-based anode and an air
cathode, contacted by non-aqueous electrolyte. The arrangement is
similar to an aqueous Li-air battery. Porous carbon generally acts as
cathode material supporting catalyst particles. Lithium in the anode
undergoes a redox reaction, and lithium ions (Li+) are
constantly transported through the electrolyte to the cathode and react
with oxygen molecules. Lithium oxide (Li2O) and lithium
peroxide (Li2O2) are generated in the air cathode.
The general reaction are presented as: [7]

Anode Reaction:

Li(s) ⇔ Li+ + e-

Cathode Reaction:

Li+ + e- + 0.5 O2
⇔ 0.5 Li2O2

Cathode Reaction:

Li+ + e-
⇔ 0.5 Li2O

Despite the promising specific energy, Li-air
batteries fall behind in several performance parameters. The highest
reported achieved specific energy is 362 Wh/kg. [7] Although it is 100%
higher than Li-ion batteries (~180Wh/kg), it only achieves 21% of the
expected practical value. [8] The specific power is ~0.46mW/g which is
only 10% of present Li-ion batteries and remains a major hurdle for
electric vehicle propulsion. Life cycle is another important concern, as
Li-air batteries degrade twice after first 50 cycles. [7] Development
cycle of Li-air batteries is reviewed to be similar to that of Li-ion
batteries. With essential parameters in automotive propulsion to be
addressed in the future, the full transition to Li-air batteries might
follow a similar path like Li-ion batteries which is 35 years of
research and development. [9]

Present Challenges

Although Li-air batteries present promising prospects
of future road electrification, various limitations exist and remain
major hurdles for this transition.

Li-air batteries are further categorized by
electrolyte, namely, aqueous and non-aqueous. The estimated energy
density is 1,300 Wh/kg for alkaline aqueous electrolyte and 1,400 Wh/kg
for acidic aqueous electrolyte. [7] However, aqueous electrolyte
contacts with Li anode and induces the redox reaction between lithium
and water/acid, which speeds up the consumption of both lithium anode
and electrolyte. Energy density of non-aqueous Li-air batteries is
predicted to be 2,790 Wh/kg, and battery cells are terminated by air
cathode being clogged by precipitated lithium oxides which are insoluble
in electrolyte. [7] Recent research suggests that a combination of both
types of electrolyte may provide a solution to both limitations, such
as, adding additives to dissolve lithium oxides or compound electrolytes
to reduce permeability between anode and electrolyte. The current energy
density of Li-air batteries are noticeably lower than the values stated
above, which is mainly resulted from the substantial weight percentage
of electrolyte per cell (~70%) while both lithium and carbon content is
only ~11%. [7]

Power density is an essential parameter is electric
propulsion. Despite the high energy density, Li-air batteries are low in
power density. During discharging process, oxygen is reduced to formed
lithium-oxides, and the charging cycle reverses chemical reaction and
produces oxygen gas. Both processes take place in the cathode surface.
As a result, to ensure a satisfactory power output, a high surface area
of cathode is substantial. [7] Rough estimation of a prototype Li-air
battery shows that, with 100 kW power output and 1mA/cm2
current density at 2.5V requires an internal surface area of 4000
m2.

Li-air batteries fall short in round-trip efficiency
which represents the ratio of energy discharged to energy needed during
charging. Typical round-trip efficiency qualifying for electric
propulsion is set at 90%. [9] However, the round-trip efficiency of
Li-air batteries with pure carbon cathode is only 57%. [7] Although this
record is further improved by cathode with platinum/gold catalysts
(PtAu/C) to 73%, it is considered not realistic in commercialization of
Li-air batteries since both platinum and gold are of extremely high
price. [10]

Lifespan is another parameter when evaluating future
potential of Li-air batteries as an electric propulsion source. Lithium
oxides form during discharging cycle as lithium ions are transferred to
the cathode and react with incoming oxygen. The recharging process
involves the reduction of lithium oxides(Li2O and
Li2O2). However, Li2O is not
electrochemically active and subsequently not participating reversible
reactions. The Li2O content varies from 0-100%, depending on
types of electrolyte and carbon cathode. [7] The whole battery cell is
terminated by accumulating lithium oxides clogging all pore volume of
the cathode, since the clogging effect prevents further transports of
both lithium ions and oxygen molecules.

Most research at present measure battery performance
under high purity oxygen environment. In actual operation, as oxygen is
extracted from ambient air, the moisture content in the air is also
transported into the battery and reacts with lithium anode, which
shortens battery lifespan. Recent research by Pacific Northwest National
Laboratory (PNNL) employed membrane as a filtration interface enabling
oxygen diffusion and moisture elimination, and operated in ambient
condition with 21% oxygen and 20% of relative humidity. The results were
encouraging, demonstrating an energy density of 362 Wh/kg over one month.
[11]

Conclusions

Although Li-air batteries demonstrate an encouraging
potential of being the future electric propulsion source, various
limitations remain to be tackled before the full transition. With a
promising high specific energy storage capacity, many challenges are to
be overcome in the future research and development of Li-air batteries.